Hi
my gs300 has a speed related vibration which i investigated using the info on this site.....it turns out that both my fount wheels have flat spots!!!
i have only had the car a fews weeks and when i bought it it had 4 brand new 245/45/18s on.
So have a look at the inner rim by spinning the wheel. The damage on my wheels was not visable from the outer rim.
Hope this helps
Steve